Enhancement: Fetch relevant K8s Secrets using a predefined label

[sigalsax]

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Current solution: The Secrets Provider code looks at the K8S_SECRETS environment variable in each of the pod manifests to know which K8s Secret to update with Conjur values.

Listing each K8s Secret in this way is not a scalable solution, especially for customers with hundreds of K8s Secrets.

Enhanced solution: We will offer the customer an additional option to define which K8s Secrets values are hosted in Conjur by defining them in terms of a label instead of individually. They will define the label that is attached to the K8s Secret and add it under K8S_SECRETS_LABEL environment variable in the Secrets Provider manifest.

Solution design: https://github.com/cyberark/secrets-provider-for-k8s/pull/137